Bottom Characteristics of an Ocean Disposal Site off Honolulu, Hawaii:
Time-based Navigational Trackline Data Managed by Routes and Events

ABSTRACT

Mamala Bay, offshore of Honolulu, Hawaii, has been a disposal area for
dredged materials from nearby Pearl and Honolulu Harbors for more than
a century. The U.S. Geological Survey, the U.S. Army Corps of Engineers,
and the U.S. Environmental Protection Agency are studying the dredged materials
and their impact on the marine environment. Data collection includes geophysical
profiling and imaging, bottom video and still photography, sediment sampling,
analyses of chemical and physical sediment properties, and evaluation of
the nature of and impact on the benthic fauna. Synthesis of this information
begins with the establishment of a geographic framework for the data.

Ship-based oceanographic surveys typically operate several data-collection
systems: navigation, bathymetry, camera, seismic-reflection, sidescan sonar,
and physical sampling. All data gathered during the survey are geographically
located by referencing their time of collection with the time of a navigational
"fix" from satellites (GPS) and/or transponders. After the field
activity is completed, time is also used to annotate interpreted data,
for example, geologic structures from seismic-reflection profiles or sidescan-sonar
images, or seafloor features from photographs. With time as the measure,
routes and related events are used to extract and analyze data along segments
of tracklines (line events) or at discrete points (point events). In addition
to enabling time as the primary measure, route treatment of tracklines
preserves the start-to-end continuity that is ordinarily fragmented by
building arc topology. Depending on the data set, the event tables may
be converted to standard line and point coverages.

Oriented bottom photographs that are time-referenced are one product
of this study. Bottom current direction and bottom roughness were determined
from the appearance of the seafloor in each photograph. A plot of the inferred
current directions as a function of time along the camera tracklines reveals
a complex pattern with a dominant northwest-trending swath through the
area. Sediment textures were determined from the photographs and also from
sample grain-size analyses. These textures provide confirmation, along
survey tracks, of the textures interpreted from the sidescan-sonar image
of the area. Correlation of observed sediment textures with image data
paves the way for construction of a sediment map of the whole area, both
along- and between-track.